Method for silicon nitride precursor solids recovery

ABSTRACT

Method and apparatus are provided for collecting reaction product solids entrained in a gaseous outflow from a reaction situs, wherein the gaseous outflow includes a condensable vapor. A condensate is formed of the condensable vapor on static mixer surfaces within a static mixer heat exchanger. The entrained reaction product solids are captured in the condensate which can be collected for further processing, such as return to the reaction situs. In production of silicon imide, optionally integrated into a production process for making silicon nitride caramic, wherein reactant feed gas comprising silicon halide and substantially inert carrier gas is reacted with liquid ammonia in a reaction vessel, silicon imide reaction product solids entrained in a gaseous outflow comprising residual carrier gas and vaporized ammonia can be captured by forming a condensate of the ammonia vapor on static mixer surfaces of a static mixer heat exchanger.

GOVERNMENT INTEREST

This invention was made with Government support under Contract No.86X-22001C (Martin Marietta Energy Systems, Inc., ORNL) awarded by theDepartment of Energy.

FIELD OF THE REACTION

The present invention relates to collecting reaction product solidsentrained in a gaseous outflow from a reaction site. The invention isparticularly useful in the recovery of silicon nitride precursor solidsentrained in a gaseous outflow of ammonia and inert carrier gas from areactor vessel containing liquid ammonia and receiving a sustained flowof silicon halide reactant vapor and inert carrier gas.

BACKGROUND OF THE INVENTION

In a chemical production process in which reaction product solids areformed in a reactor vessel, a gaseous outflow from the reactor vesselmay be exhausted to accommodate a continuing inflow of reactant feed gasduring the reaction, the feed gas possibly including a large volume ofinert carrier gas. Reaction product solids may become entrained in thegaseous outflow, particularly where agitation of the reactants isprovided. If the portion of the reaction product entrained in thegaseous outflow is not recovered, the effective yield of the reaction isreduced, productivity is decreased and production costs are increased.

Reduced product yield through loss of reaction product solids entrainedin a gaseous outflow from the reactor vessel is encountered, forexample, in a known production process for silicon nitride precursor.Silicon nitride-based ceramics are considered amongst the toughest ofthe monolithic ceramics for use above 1000° C. The toughness is thoughtto be due primarily to a high degree of grain interlocking which can bedeveloped from appropriate powders. Silicon nitride-based ceramics canbe formed to near-net shape in a pressureless sintering operation frompowders having the necessary characteristics. Silicon nitride ceramicsare, therefore, prime candidates for light weight engine components, forexample, in which toughness is needed together with high temperaturewear resistance.

The above mentioned method for making silicon nitride precursor istaught in U.S. Pat. No. 4,732,746 to Crosbie et al. The Crosbie et alpatent is directed particularly to production of silicon imide solids asa silicon nitride precursor. To prevent or reduce carbon contamination,an inert carrier gas, preferably nitrogen or argon, is used to bringsilicon halide, preferably SiCl₄, vapor into contact with liquidammonia. The reaction produces a mixture of precipitated silicon imidein liquid ammonia having dissolved ammonium halide. The silicon halidevapor is brought into reaction with the liquid ammonia by means ofproviding a sustained inflow to the reaction situs of a reactant feedgas comprising silicon halide vapor and the inert carrier gas. A gaseousoutflow comprising primarily residual carrier gas and a certain amountof vaporized ammonia is released from the reaction situs to accommodatethe continuing inflow of fresh reactant feed gas. The reaction situspreferably is agitated during the reaction. A certain fraction of thesilicon nitride precursor precipitate may be entrained in the gaseousoutflow from the reaction situs. The loss of such entrained solidsreduces the effective reaction yield.

It would be desirable in numerous reaction schemes and productionprocesses, including particularly, for example, the production ofsilicon nitride precursor in accordance with the Crosbie et al patent,wherein reaction product solids become entrained in a gaseous outflowfrom a reaction situs, to recover such reaction product solids. This andother objects and advantages of the present invention will be betterunderstood from the following disclosure and discussion of theinvention.

SUMMARY OF THE INVENTION

In accordance with the invention, method and apparatus are provided forcollecting solids, such as reaction product solids, entrained in a gasflow comprising condensable vapor. In the case of a gaseous outflow froma reaction situs during a reaction, the gaseous outflow comprises vaporcondensable at a reduced temperature, that is, at a condensationtemperature lower than the temperature at which it exits the reactionvessel. The gaseous outflow is passed from an outlet of the reactorvessel to a static mixer heat exchanger. The static mixer heat exchangerhas a static mixer cooling chamber defining a flow path for the gaseousoutflow between the inlet and an outlet. Static mixer surfaces areprovided in the flow path of the static mixer cooling chamber forcapturing the entrained solids in a condensate formed of the aforesaidcondensable vapor. Specifically, the static mixer heat exchanger furtherprovides cooling means for maintaining the static mixer surfaces at atemperature not greater than the aforesaid reduced temperature at whichthe condensable gas condenses. Entrained reaction product solids in agaseous outflow contact, and are thereby captured in, the condensateformed on the static mixer surfaces. The condensate and the reactionproduct solids captured therein can be collected, for example, forreturn to the reactor vessel or for other processing.

The present invention provides advantages in collecting or recoveringreaction product solids which may otherwise be lost or which wouldotherwise require different, more expensive or difficult collectionmethods and apparatus. Such collection of reaction product solids willin many instances increase the effective reaction product yield, therebyimproving the efficiency of the production process.

In addition, in those embodiments of the invention wherein thecondensate formed in the static mixer heat exchanger comprises reactantfor the ongoing reaction, the return of the condensate, with therecovered reaction product solids, can further improve the efficiency ofthe production process. This is particularly true where the reactant inthe condensate would otherwise be lost or require more expensive ordifficult collection and return apparatus.

The method and apparatus of the invention are particularly advantageousin preferred embodiments of the invention, discussed further below,wherein silicon nitride precursor solids are entrained in a gaseousoutflow from a reaction situs during an ongoing reaction. The staticmixer heat exchanger can treat gaseous outflow from the reaction situsanaerobically, including steps of receiving the gaseous outflow, forminga condensate of a condensable vapor in the gaseous outflow to captureentrained solids, and returning the condensate, optionally throughintermediate fluid communication means, to the reaction situs or otherreceiving point for further processing. In such preferred embodimentsthe invention is particularly advantageous in that the entrained solidscomprise reaction product, thus directly enhancing process yield, andthe condensate returned with the solids to the ongoing reaction isusable reactant.

These and additional features and advantages will be better understoodin the light of the following detailed description of certain preferredembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of apparatus in accordance with apreferred embodiment of the invention for carrying out a reaction forthe production of silicon nitride precursor solids, wherein precursorsolids are recovered and returned anaerobically to the reaction situs ina condensate of reactant vapor formed by a static mixer heat exchangertreating a gaseous outflow from the reaction situs.

FIG. 2 is an enlarged perspective view of a static mixer heat exchangersuitable for use in the apparatus of the preferred embodiment of FIG. 1.

FIG. 3 is an enlarged perspective view of the static mixer meansdisposed in the flow path of the cooling chamber of the static mixerheat exchanger of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method and apparatus of the invention for recovering reactionproduct solids entrained in a gaseous outflow from a reaction situs isdescribed below in connection with certain particularly preferredembodiments of the invention involving the production of silicon nitrideprecursor solids. It will be understood by those skilled in the art,however, that the invention has broader application in accordance withthe general principles illustrated by such preferred embodiments. Inparticular, the invention is illustrated below in connection with themethod and apparatus for making silicon nitride precursor disclosed inU.S. Pat. No. 4,732,746 to Crosbie et al, which disclosure isincorporated herein by reference, as noted above. More specifically, theparticularly preferred embodiment of the invention now describedinvolves a low temperature reaction between silicon tetrachloride vaporand liquid ammonia under pressure. Ammonium chloride triammoniate isproduced along with the desired silicon diimide reaction product solids.The silicon diimide polymerizes and precipitates from the ammoniasolution. The ammonium chloride triammoniate remains soluble and isseparated typically by repeatedly washing the polymeric silicon diimidewith liquid ammonia. The polymeric silicon diimide may then be dried byevaporating the ammonia.

In that regard, while not wishing to be bound by theory, the followingdiscussion will assume the following reaction in the production of suchsilicon nitride precursor solids:

    SiCl.sub.4 +18 NH.sub.3 →1/x [Si(NH).sub.2 ].sub.x +4 NH.sub.4 Cl.3NH.sub.3

wherein x typically is understood to have a value much greater than oneto designate the polymeric nature of the diimide. The silicon imideprecursor solid, specifically, the polymeric silicon imide, is listed inChemical Abstracts under number 29696-97-7 (Silanediimine, homopolymer),although there may also be a listing at Chemical Abstracts number17022-99-0 (Silanediimine, monomer). Silicon nitride is formed bydecomposition of the precursor, preferably by thermal decompositionperformed anaerobically and in the absence of chlorine. Again, while notwishing to be bound by theory, the reaction chemistry is understood tobe:

    Si(NH).sub.2 →1/3 Si.sub.3 N.sub.4 +2/3 NH.sub.3

Nitrogen and hydrogen gases may replace the ammonia at hightemperatures.

The following glossary is provided for convenience of reference in thefollowing discussion.

Ammonia; anhydrous ammonia; NH₃

1) compound: Chem. Abstracts Reg. No. 7664-41-7

2) liquid ammonia

Ammonium chloride; by-product chloride; wastage (certain instances); NH₄Cl

1) compound: Chem. Abstracts Reg. No. 12125-02-9

Ammonium chloride triammoniate; NH₄ Cl.3NH₃

1) compound: Chem. Abstracts Reg. No. 12394-36-4

Decomposition product; calcine product; product; final product

1) silicon nitride powder recovered after high temperature decompositionof imide intermediate

Intermediate product; reaction product; product (certain instances);imide intermediate; silicon nitride precursor; silicon diimide; imide;silanediimine; [Si(NH)₂ ]_(x)

1) compound: Chem. Abstracts Reg. No. 17022-99-0 or Reg. No. 29696-97-7

2) powder product from reaction of silicon halide, e.g., SiCl₄, with NH₃; can be thermally treated to form silicon nitride

Silicon nitride; nitride; Si₃ N₄

1) compound: Chem. Abstracts Reg. No. 12033-89-5

2) composition of matter with the compound as principal constituent andtetrahedral bonding similar to diamond

3) a polycrystalline ceramic with an interlocking microstructure formedduring thermal processing which leads to high toughness and hardness

Silicon tetrachloride; tetrachlorosilane; SiCl₄

1) compound: Chem. Abstracts Reg. No. 10026-04-7

Vapor-SiCl₄ --liquid-ammonia process; vapor-liquid process

1) process in which SiCl₄ vapor in an inert carrier gas contacts liquidammonia.

As noted above, the present invention is especially advantageous in theproduction of silicon nitride precursor solids. In one particularlypreferred embodiment of such application of the invention, siliconhalide, most preferably silicon tetrachloride, is used as a firstreactant due to its relatively low cost, commodity status and therelative ease of purification and by-product separation in the reactionscheme. In fact, it is a special feature of the vapor-chloride --liquid-ammonia process that the silicon reactant is input to the reactorvessel in the vapor form. Specifically, a saturator can be used toconvert liquid silicon tetrachloride to the vapor form in mixture withan inert carrier gas, preferably argon or nitrogen. A controlled flowrate carrier gas stream can be saturated with SiCl₄ by bubbling itthrough the liquid at or slightly below room temperature. The resultantgas stream is then diluted with a bypass flow stream (that is, anadditional feed of carrier gas which bypasses the saturator) to form afinal reactant feed gas stream which is slightly undersaturated at thereactor temperature. This undersaturation can aid in avoidingcondensation of liquid SiCl₄ at the reactor inlet. The nitrogen or othercarrier gas streams preferably are controlled by mass flow controllersessentially independent of system pressure. During operation, a slightdrop in saturator temperature may be observed, reflecting the heat ofvaporization of the SiCl₄.

The SiCl₄ feed stock preferably is purified through distillation,chelation or absorption for volatile dissolved chlorides, and submicronfiltration of particulate matter. Suitable techniques are taught, forexample, in J. W. Mitchell and J. E. Kessler, "Purification of OpticalWave Guide Glass Forming Reagents: Phosphorous Oxychloride," J.Electrochem. Soc., 131 [2] 361-65 (1984), which teaching is incorporatedherein by reference. There is also, effectively, an additionaldistillation in the saturator. There is also an opportunity for finalmicrofiltration of entrained fine solid chlorides and oxides beforeentry of the reactant feed gas into the reactor vessel. It is within theability of those skilled in the art to determine a suitable volume ofcarrier gas, which is a function of the vapor pressure and the totalsystem pressure. The volume is calculated as part of the overall massbalance of the reaction scheme. Typically, operation of the reactorvessel above -20° C. is preferred to limit the amount of carrier gasrequired. The carrier gas preferably is nitrogen derived from liquidnitrogen. Typically, the oxygen and hydrocarbon purity of such carriergas is sufficient for use for reactor temperatures close to roomtemperature.

Notwithstanding that the SiCl₄ --NH₃ reaction is ordinarily exothermic,the "vapor-chloride -- liquid-ammonia" process can be net endothermic orheat-neutral. The exothermic chloride-ammonia reaction can be more thanoffset at 0° C. by latent heat of vaporization of NH₃ into the carriergas. By operation at temperatures near room temperature, the netendotherm can approach zero, thereby minimizing potential problems inscaling the production process. In any event, those skilled in the artwill recognize that control of reaction heat yield is important forprocess scale-up, since heat transfer for a given reactor geometry is afunction of heat transfer surface to volume ratio. In this regard, theteachings presented in G. M. Crosbie et al, "Synthesis of High PuritySinterable Si₃ N₄ Powders", Oak Ridge National Laboratory PublicationORNL/Sub/85-SB012/1, which is available from the National TechnicalInformation Service, U.S. Department of Commerce, is incorporated hereinby reference. Most significant in this regard, with respect to thepreferred embodiment of the invention under discussion, is that the useof an inert carrier gas in the silicon halide reactant feed gas assistsin maintaining the reactor at or below about room temperature by thelatent heat of vaporization of liquid ammonia into the carrier gas. Thatis, as noted above, the latent heat of vaporization of ammonia toestablish its partial pressure in the carrier gas can substantiallyoffset the exothermic heat of the imide-forming reaction. As explainedfurther below, the presence of condensable ammonia vapor in the gaseousoutflow of residual carrier gas from the reactor vessel plays a criticalrole in recovering silicon nitride precursor solids entrained in thegaseous outflow.

Referring now specifically to the drawings, the apparatus 10 shownschematically in FIG. 1 includes a reactor vessel 12 comprising agitatormeans 14. As now described in detail, the apparatus, and the method forits use, are for producing silicon nitride precursor powders,specifically, silicon diimide, Si(NH)₂. A quantity of liquid ammonia isfed through line 16 to reactor vessel 12. In the embodiment illustrated,valve 17 is opened, valve 18 is closed and the proper amount of ammoniais charged to ammonia metering tank 19. Thereafter, valve 17 is closedand valve 18 is opened, allowing the liquid ammonia to run through line16 to vertical column or stack 20 to reactor vessel 12. Those skilled inthe art will recognize alternative suitable means for metering a propercharge of liquid ammonia to the reactor vessel 12. Thus, for example,rather than ammonia metering tank 19, weighing means for measuring theliquid ammonia charge may be employed in accordance with devices andmethods known in the art.

Prior to charging liquid ammonia to the reactor vessel, the systempreferably is evacuated. This is found to improve the purity of theproduct by reducing contaminants in the system. For this purpose, vacuumline 11 is connected to a vacuum pump. Valve 13 is opened duringevacuation pumping and subsequently closed prior to charging ammonia tothe reactor vessel 12.

Liquid SiCl₄ is provided in saturator 22. To establish a correct andsteady flow of the silicon chloride reactant feed gas, valve 23 isclosed and valves 24, 25 and 26 are opened. Nitrogen or other inertcarrier gas is fed through line 27. Silicon tetrachloride vapors aregenerated as the carrier gas bubbles through the saturator, which is apacked bed filled with liquid silicon tetrachloride. The silicontetrachloride preferably is of electronic grade, available, for example,from Solkatronic Chemicals, Inc., Fairfield, N.J. The stream from thesaturator flows to a mixing chamber 21 above the saturator, where it iscombined with a by-pass gas flow, that is, additional carrier gas, toprevent supersaturation at the lower temperature of the reactor.Preferably, the saturator is operated at room temperature to preventmoisture condensation on the separator from ambient air. This reducesthe possibility of corrosion due to the mixture of condensed moisturewith any leakage of silicon chloride from the separator. Carrier gas,saturated with the silicon chloride or other silicon halide reactant,may be mixed with an additional portion of carrier gas via line 28. Theamount of additional carrier gas should be sufficient to prevent anysubstantial condensation of silicon chloride at the reactor vesseloperating temperature. When a steady flow has been established, its flowto wastage is stopped by closing valve 24 and substantiallysimultaneously opening valve 23.

The reactant gas feed line 29 preferably includes a downwardly extendingtube within reactor vessel 12 or other means for discharging thereactant feed gas below level 30 of the liquid ammonia previouslycharged to the reactor vessel. In this regard, the teachings of U.S.Pat. No. 4,196,178 are incorporated herein by reference. This feature isfound to substantially eliminate solids formation at the end of thereactant gas feed line and to provide good reactant mixing, especiallyin view of the agitator means 14 preferably operating within the reactorvessel 12.

The reactant feed gas preferably is fed to the reactor vesselcontinuously over a period of time as the reaction to produce thesilicon diimide is ongoing. From the above discussion it will beappreciated that the reactor vessel operates under pressure, such thatthe ongoing flow of reactant feed gas causes the aforesaid gaseousoutflow from the reactor vessel. Those skilled in the art will alsorecognize that operation of the reactor under pressure is particularlyadvantageous in the context of the preferred embodiment of the inventionillustrated in FIG. 1 for the production of silicon nitride precursor.In that context, the reactor vessel preferably is operated at or above35 psig, more preferably in the range of 35 to 250 psig.

The carrier gas is exhausted from the reactor vessel via vertical stack20. As noted above, a certain amount of liquid ammonia vaporizes intothe carrier gas during the reaction. The gaseous outflow from thereaction situs during the reaction, in addition to comprising residualcarrier gas and vaporized ammonia, also has been found to entrain asubstantial amount of silicon diimide solids being formed in thereaction. Loss of this reaction product would reduce the effective yieldof the production process. In accordance with the present invention thisloss is substantially reduced. Specifically, the presence in the gaseousoutflow of a condensable vapor, that is, ammonia vapor, isadvantageously employed to recover the entrained silicon diimide solids.More specifically, a static mixer heat exchanger 32 is included invertical stack 20 through which the gaseous outflow passes from thereactor vessel 12.

Static mixer heat exchangers are known to those skilled in the art.Static mixers are understood by those skilled in the art to be deviceswherein fluid media are forced to mix themselves through a progressionof divisions and recombinations, typically with 2^(n) layerings per nelements. Such devices typically require no moving parts and,accordingly, maintenance and operating costs typically are extremelylow. Control means typically are not required for controlling fluid flowthrough a static mixer. The energy for mixing or, in the case of thepresent invention, for bringing the gaseous outflow into contact withthe static mixer surfaces (as further described below), is provided bythe pressure under which the fluid flows through the static mixer.Typically, the pressure drop across the static mixer is low. Staticmixer heat exchangers may employ a water jacket, as seen in theembodiment of FIG. 2, for example, although alternative embodiments mayemploy cooling fluid channels in the gas flow path, e.g., through theinterior of the static mixer elements. Static mixer heat exchangerssuitable for the present invention are commercially available, forexample, from Kenics Corp., North Andover, Mass. Static mixers arediscussed in Chemical Engineers' Handbook, 5th Edition, McGraw-Hill BookCompany, §19, p. 32.

In the preferred embodiment of FIGS. 1-3, the static mixer heatexchanger 30 comprises a static mixer cooling chamber 32 which defines aflow path for the gaseous outflow in the direction of arrow 24 betweeninlet 36 and outlet 38. The static mixer heat exchanger furthercomprises static mixer surfaces 40 disposed in the flow path 32. As bestseen in FIG. 3, the static mixer surfaces preferably consist ofalternate-hand helix-approximating elements juxtaposed at 90° to oneanother in series in the flow path. Alternative suitable designs for thestatic mixer surfaces 40 within the cooling chamber 32 of the staticmixer heat exchanger 30 will be apparent to those skilled in the art inview of the present disclosure.

The static mixer heat exchanger 30, as illustrated in FIG. 2, mayfurther comprise a cooling jacket 44. Cooling fluid, such as water ormore preferably anhydrous alcohol, is fed via inlet 46 to surround thestatic mixing cooling chamber 32, exiting via outlet 48. Fluid feed andexhaust lines (not shown) can be connected to inlet 46 and outlet 48 ina usual manner.

Vertical stack 20 is seen to further comprise fluid communication means50 for communicating gaseous outflow from the reactor vessel to thestatic mixer heat exchanger 30. Fluid communication means 50 comprises avertical tubular conduit with a view port 52.

In operation, during the production of silicon diimide, reactant feedgas enters in the manner described above via feed tube 29 and gaseousoutflow comprising residual carrier gas, ammonia vapor and entrainedreaction product solids are exhausted from the reactor vessel viavertical stack 20. In passing through the cooling chamber of staticmixer heat exchanger 30, the liquid ammonia is condensed on the staticmixer surfaces. As best seen in FIG. 2, the static mixer surfaces are incontact with the inner shell which defines the flow path for the gaseousoutflow. Being heat conductive, the static mixer elements are at reducedtemperature by operation of the cooling means of the static mixer heatexchanger, specifically, the cooling jacket 44. Those skilled in theart, in view of the present discussion, will recognize that referenceherein to operation of the static mixer heat exchanger at a "reducedtemperature" means that the static mixer heat exchanger, morespecifically the static mixer surfaces disposed within the coolingchamber thereof, are maintained at a temperature less than that at whichthe gaseous outflow exits the reaction vessel. More specifically, the"reduced temperature" is low enough to condense onto the static mixersurfaces at least one condensable vapor or gas comprising the gaseousoutflow from the reactor vessel. In the case of the preferred embodimentof FIG. 1, wherein the gaseous outflow comprises nitrogen or other inertcarrier gas and ammonia vapor (along with the entrained reaction productsolids) the temperature should be at or below the condensationtemperature of the ammonia. Since the vapor pressure is a function oftemperature, the preferred condensor temperature depends upon thereactor temperature used in a particular operation. To minimize ammonialoss through the static mixer heat exchanger, the preferred condensortemperature is at least 20° C. below that of the reactor exittemperature. That is, the static mixer surfaces preferably aremaintained at or below that "condensor temperature". It will be apparentto those skilled in the art that in alternative embodiments of theinvention the requisite temperature will vary, depending on thecondensation temperature of the condensable gas or gases of which aparticular gaseous outflow is comprised.

In the particularly preferred embodiment illustrated in FIG. 1, thecondensate, with reaction production solids captured therein, iscollected simply by being returned to the reactor vessel. Those skilledin the art will recognize that the condensate and solids may be"collected" by either a batch or continuous flow return to the reactionsitus or to some other place for additional processing, etc. Thevertical arrangement of stack 20 (and other arrangements offset fromvertical, but at an elevation above the reactor vessel) allows thereturn flow to the reactor vessel to be accomplished by simple action ofgravity upon the condensate. Thus, the preferred embodiment illustratedrepresents a particularly elegant advance in the art, especially in thatno control means or moving parts are required for collecting andreturning the recovered solids to the reactor vessel, and also in thatthe condensate is a reactant and, therefore, is also advantageouslyreturned to the reactor vessel. In view of the present discussion, thoseskilled in the art will appreciate that in various embodiments of theinvention the condensate with captured solids therein may not bereturned to a reactor vessel but, rather, may be collected separately.Thus, for example, means may be provided for diverting the flow ofcondensate to a separate holding tank for further processing.

Upon exiting the static mixer heat exchanger, the gaseous outflow passesthrough condenser 60. Condenser 60 removes additional liquid ammonia andperhaps trace reaction product solids from the residual carrier gas.Optionally, the condenser may comprise an additional static mixer heatexchanger, although the economic advantage of an additional static mixerheat exchanger will depend upon the particular production processinvolved. Any additional ammonia which is condensed, and any tracereaction product solids captured thereby, may be returned downwardlythrough stack 20 by force of gravity. The residual carrier gas exitingvia line 62 at the top of condenser 60 is passed to backpressure valve54, which may also comprise a filter. In the embodiment of FIG. 1, thebackpressure valve 54 may be employed to control overall systempressure. Line 56 exiting backpressure valve 54 typically will exit to ascrubber or the like and then to the atmosphere.

Valve 64 is normally closed during the reaction. Upon completion of thereaction, the reactant feed gas is stopped. Agitation is stopped andsilicon diimide solids settle in the reactor vessel. Supernatant liquidmay be decanted from the reactor vessel via decantation line 70 towastage tank 72 by opening valve 64. The rate of decantation can becontrolled by controlling pressure differential between the wastage tank72 and the reactor vessel 12 via pressure equalization line 74. Thedepth of dip tube 65 is adjusted to select the proportion of supernatantwithdrawn and the fraction of reaction product solids carried off to thewastage tank 72. Wastage tank 72 is vented through waste backpressurevalve 76. In addition, decantation can be advanced by feeding nitrogenor other inert gas to the reactor vessel 12 via pressure line 78 duringdecantation. Optionally the reaction product is rinsed, followed byadditional decanting. In addition to, or as an alternative to,decantation for the separation of imide from liquid ammonia whichcontains the by-product NH₄ Cl, a decanting centrifuge may be used.Additional suitable methods for the liquids/solids separation includecentrifuge, filter, flash evaporation, etc. Valve 80 and pressure line78 are normally closed during the reaction step.

The decantation step leaves in the reactor a portion of the originalmixture which now is rich in silicon diimide precursor solids. Theremaining liquid comprises primarily unreacted NH₃ and NH₄ Clby-product. This reaction product enriched mixture is extracted from thereactor vessel for further processing to separate and purify thereaction product solids which then can be converted to silicon nitridepowder suitable for the manufacture of components, for example, motorvehicle engine components or the like. Such conversion of the silicondiimide reaction product solids to silicon nitride is typicallyaccomplished by two stage thermal decomposition according to methods andapparatus well known to those skilled in the art. The extraction of thesolids rich mixture from the reactor is accomplished, in accordance withthe preferred embodiment illustrated in FIG. 1, by actuation of cylindervalve 82 by means of ram type valve actuator 84. Opening cylinder valve82 allows the mixture to flow by gravity from reactor vessel 12 tointermediate product tank 86 via fluid communication line 88. Viewingwindow 90 in line 88 allows visual confirmation that the flow from thereactor to the intermediate product tank has been accomplished.Backpressure valve 92 fitted to intermediate product tank 86 assists incontrolling the pressure drop in the intermediate product tank 86. Theslurry is chilled by NH₃ venting to a scrubber to reduce the pressure inthe vessel and thereby induce NH₃ evaporation. The chilled mixture maythen be transferred by gravity flow to a ceramic decomposition tube viathe pair of valves 87.

The apparatus illustrated in FIGS. 1 through 3 is adapted forsemi-continuous operation in that liquid ammonia is loaded batch-wiseand the silicon halide reactant feed gas flows continuously to thereactor during the reaction. Those skilled in the art will readilyappreciate in view of this disclosure that the invention is equallyapplicable to continuous reaction processes whenever a gaseous outflowfrom the reactor vessel entrains reaction product solids and has acondensable vapor or gas species in sufficient quantity to wet staticmixer surfaces adequately to recapture such entrained solids.

The imide reaction product solids is suitable for two stage thermaldecomposition to silicon nitride, the first stage optionally beingfluidized bed processing. Fast heating on injection into a fluidized bedwould likely aid in the removable of chloride residues with leastproduct losses. The silicon imide precursor is likely used immediatelyto make silicon nitride in view of its sensitivity to exposure to airand its resultant degradation over time. Recycled gas streams could beused as fluidizing gas. Specifically, gaseous outflow form the reactorvessel, after being processed through the above mentioned static mixerheat exchanger and condenser, may be used as fluidizing gas in afluidized bed employed for thermal decomposition of silicon imidereaction product to silicon nitride. In this regard the teachings ofU.S. Pat. No. 4,859,443 to Marosi regarding a process of producingsilicon nitride in a fluidized bed of gaseous ammonia is incorporatedherein by reference. Fluidizing gas flowing from the fluid bed can bepassed to a cooler/condenser and/or spray tower to recover productsolids entrained in the gas. Liquid ammonia can be used for washing inthe spray tower. Material which might otherwise be lost product isrecovered and coarsened by a second passage through the washing stage.It appears that a minimum fluidization velocity in the fluid bed may beon the order of 2.8 cm/sec. The fluidized bed may also contain a cycloneseparator to remove silicon diimide from fluidizing gas, or othergas/solids separator means in accordance with techniques and equipmentwell known to those skilled in the art. In regard to a process ofcalcining silicon diimide to silicon nitride in a fluidized bed ofgaseous ammonia, the teaching of U.S. Pat. No. 4,859,443 to Marosi isincorporated herein by reference.

In view of the foregoing disclosure and discussion of preferredembodiments of the invention, those skilled in the art will readilyappreciate that the present invention has application to any productionprocess wherein gaseous outflow from a reactor, in which reactionproduct solids are entrained, comprises a condensable gas or vapor insufficient quantity to condense on static mixer surfaces adequately tocapture a substantial portion of such entrained reaction product solids.Thus, while preferred embodiments of the invention have been illustratedand described, it will be understood that various changes andmodifications may be made without departing from the invention, and itis intended to cover in the appended claims all such changes andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A method for collecting solids entrained in a gaseousoutflow from a reaction situs during a reaction, the gaseous outflowexiting the reaction situs at an exit temperature and comprising a vaporcondensable at a condensation temperature below the exit temperature,the method comprising:directing the gaseous outflow to an inlet of astatic mixer heat exchanger having (a) a cooling chamber defining a flowpath between the inlet and an outlet, (b) static mixer surfaces in theflow path for forming a condensate of the condensable vapor, and (c)cooling means for maintaining the static mixer surfaces at a temperaturenot greater than the condensation temperature; and operating the coolingmeans to cool the static mixer surfaces to form a condensate of thecondensable vapor on the static mixer surfaces, capturing solids fromthe gaseous outflow in the condensate.
 2. The method for collectingsolids in accordance with claim 1, further comprising the step ofpassing the condensate with reaction product solids therein back to thereaction situs.
 3. The method for collecting reaction product solids inaccordance with claim 2, wherein the condensate with solids therein ispassed from the static mixer heat exchanger to the reaction situs bygravity flow through the cooling chamber inlet.
 4. The method forcollecting reaction product solids in accordance with claim 3 whereinthe cooling means comprises a fluid-tight jacket surrounding the coolingchamber and having an inlet and an outlet for a flow of cooling fluidthrough the jacket, and operating the cooling means comprises passing aflow of cooling fluid through the jacket.
 5. The method for collectingsolids in accordance with claim 4, wherein the cooling fluid isanhydrous alcohol.
 6. The method for collecting solids in accordancewith claim 1, wherein the gaseous outflow further comprises carrier gasfed to the reaction situs in a flow of reactant feed gas at asubstantially steady rate during the reaction, the temperature at whichthe static mixer surfaces are maintained being higher than thecondensation temperature of the carrier gas.
 7. A method of collectingsilicon nitride precursor solids entrained in a gaseous outflow from areaction situs, the gaseous outflow exiting the reaction situs at anexit temperature and comprising a vapor condensable at a condensationtemperature below the exit temperature and a substantially inert carriergas not condensable at the condensation temperature, the methodcomprising:passing the gaseous outflow through a static mixer coolingchamber of a static mixer heat exchanger, the static mixer coolingchamber defining a flow path between an inlet and an outlet, the staticmixer heat exchanger further comprising static mixer surfaces in theflow path and cooling means for cooling the static mixer surfaces to atemperature not greater than the condensation temperature; operating thecooling means to form a condensate of the vapor on the static mixersurfaces to capture silicon nitride precursor solids in the condensate;and collecting the condensate and the silicon nitride precursor solidscaptured therein.
 8. The method for collecting silicon, nitrideprecursor solids in accordance with claim 7, wherein the step ofcollecting the condensate and the silicon nitride precursor solidscaptured therein comprises returning them in a substantially continuousflow to an ongoing reaction producing silicon nitride precursor solidsat the reaction situs.
 9. The method for collecting silicon nitrideprecursor solids in accordance with claim 8, wherein the reaction situsis pressurized and the gaseous outflow is passed to the static mixercooling chamber under pressure from the reaction situs.
 10. The methodfor collecting silicon nitride precursor solids in accordance with claim9, further comprising feeding reactant feed gas comprising siliconhalide vapor in the carrier gas to the reaction situs during thereaction, wherein the condensable vapor of the gaseous outflow isammonia vapor, the condensation temperature being at least 20° C. belowthe exit temperature.
 11. A method of making silicon diimidecomprising:(a) introducing liquid ammonia into a reaction vessel; (b)initiating and maintaining for a period of time a flow into the reactionvessel, at above atmospheric pressure, of reactant feed gas comprisingsilicon halide vapor in an inert carrier gas, the flow of reactant feedgas passing through a reactant feed inlet tube extending downwardly intothe liquid ammonia, to sustain an ongoing reaction between the siliconhalide and the ammonia to produce silicon diimide reaction product; (c)releasing gaseous outflow from the reactor vessel at an exit temperaturethrough a first reaction product outlet to a static mixer coolingchamber of a static mixer heat exchanger, the gaseous outflow comprisingsilicon diimide solids entrained in a mixture comprising ammonia vaporand carrier gas, the static mixer cooling chamber defining a flow pathfor the gaseous outflow between an inlet and an outlet, static mixersurfaces being disposed in the flow path, the static mixer heatexchanger further comprising a cooling means for cooling the staticmixer surfaces to form a condensate comprising liquid ammonia on thestatic mixer surfaces to capture silicon diimide solids therein; and (d)returning the condensate and captured silicon diimide solids to thereaction vessel in an ongoing flow during the reaction.
 12. The methodof making silicon diimide in accordance with claim 11 further comprisingforming the reactant feed gas by passing the carrier gas through asaturator charged with liquid silicon halide and then adding additionalcarrier gas to achieve a reduced saturation level prior to introducingthe reactant fee gas into the reaction vessel.
 13. The method of makingsilicon diimide in accordance with claim 12 wherein the silicon halideis SiCl₄.
 14. The method of making silicon diimide in accordance withclaim 11 wherein the cooling means is a cooling jacket surrounding atleast a substantial portion of the flow path, the cooling jacket havingan inlet and an outlet for a flow of cooling fluid.
 15. The method ofmaking silicon diimide in accordance with claim 14 wherein alcohol isflowed through the cooling jacket to maintain the temperature of thestatic mixer surfaces at least 20° C. below the exit temperature. 16.The method of making silicon diimide in accordance with claim 11 whereinthe carrier gas is selected from the group consisting of nitrogen andargon.
 17. The method of making silicon diimide in accordance with claim11 wherein the static mixer surfaces comprise surfaces of alternate-handhelical elements positioned in series in the flow path, each saidelement being positioned at about 90° to adjacent others.
 18. The methodof making silicon diimide in accordance with claim 11 further comprisingagitating the reactants during the reaction.
 19. The method of makingsilicon diimide in accordance with claim 11 further comprising stoppingthe flow of reactant feed gas, followed by liquid/solids separation of areaction product mixture in the reactor vessel to separate silicondiimide solids from at least a portion of residual liquid comprisingunreacted ammonia and ammonium halide dissolved therein.
 20. The methodof making silicon diimide in accordance with claim 19 wherein the molarratio of liquid ammonia to the ammonium halide in the reaction productmixture is in the range of 18 to
 200. 21. The method of making silicondiimide in accordance with claim 19 wherein the liquid/solids separationcomprises decanting an upper portion of the residual liquid from thereactor vessel and passing a remaining portion thereafter from thereaction situs to a secondary liquid/solid separation step.
 22. Themethod of making silicon diimide in accordance with claim 21 whereinsaid passing of the remaining portion of the reaction product mixturefrom the reaction situs is performed anaerobically and comprisesgravitational concentration of silicon nitride solids in a conical zonefollowed by a transfer from the conical zone along with a reducedportion of the residual liquid to a controlled atmosphere furnace. 23.The method of making silicon diimide in accordance with claim 11 whereinthe reaction is carried out in the reactor vessel under pressure of atleast 35 psig.
 24. The method of making silicon diimide in accordancewith claim 23 wherein the pressure is between 35 and 250 psig.
 25. Themethod of making silicon diimide in accordance with claim 11 wherein thereaction is controlled to provide a neutral or net endothermic heattransfer between the carrier gas and the liquid ammonia in the reactorvessel by offsetting exothermic heat of reaction of silicon halide withammonia by latent heat of evaporization of ammonia into the carrier gas.26. The method of making silicon diimide in accordance with claim 25wherein the temperature of the reaction situs during the reaction ismaintained in the range of -20° to +40° C.
 27. A method of makingsilicon nitride comprising:(a) reacting silicon halide vapor with liquidammonia at a reaction situs in an inert environment having a pressure ofat least 35 psig, which environment is effectively devoid of organiccontaminants, the reaction producing a mixture of precipitated siliconimide in liquid ammonia having dissolved ammonium halide; (b) passing agaseous outflow comprising precipitated silicon imide solids entrainedin a mixture of ammonia vapor and substantially inert carrier gasthrough an outlet port from the reaction situs to an inlet of a staticmixer cooling chamber of a static mixer heat exchanger, the static mixercooling chamber defining a gas flow path between the inlet and anoutlet; (c) collecting silicon imide solids captured in liquid ammoniacondensed from ammonia vapor in the gaseous outflow on static mixersurfaces within the gas flow path by operating static mixer coolingmeans of the static mixer heat exchanger for cooling the static mixersurfaces to a temperature not greater than the condensation temperatureof the ammonia vapor; (d) returning the silicon imide solids captured inliquid ammonia to the reaction situs; (e) extracting a portion of saidmixture from the reaction situs; (f) separating liquid from siliconimide solids in said portion; and (g) thermally decomposing the siliconimide solids to silicon nitride.
 28. A method of making silicon nitridecomprising:(a) charging liquid ammonia to a reactor vessel having areactant feed gas inlet port and a gaseous outflow outlet port; (b)introducing a flow of silicon halide vapor in a substantially inertcarrier gas into the liquid ammonia in the reactor vessel under pressurewhile providing agitation to the liquid ammonia; (c) controllingreaction between the silicon halide and the liquid ammonia at thereaction situs to provide a neutral or net endothermic heat transfertherebetween, at least in part, by balancing the exothermic reactionagainst latent heat of vaporization of ammonia into the carrier gas; (d)passing a gaseous outflow comprising silicon diimide solids entrained ina mixture of ammonia vapor and carrier gas through the outlet port to aninlet of a static mixer cooling chamber of a static mixer heatexchanger, the static mixer cooling chamber defining a flow path betweenthe inlet and an outlet; (e) collecting silicon imide solids captured inliquid ammonia condensed from ammonia vapor in the gaseous outflow onstatic mixer surfaces within the gas flow path by operating static mixercooling means of the static mixer heat exchanger for cooling the staticmixer surfaces to a temperature not greater than the condensationtemperature of the ammonia vapor; (f) returning the silicon imide solidscaptured in liquid ammonia to the reaction situs; (g) stopping the flowof silicon halide vapor to the reactor vessel and anaerobicallytransferring silicon imide solids in residual liquid to a liquid/solidsseparation situs and there separating liquid from the silicon imidesolids; and (h) thermally decomposing the silicon imide solids tosilicon nitride.
 29. The method of making silicon nitride in accordancewith claim 28 wherein the silicon imide is thermally decomposed tosilicon nitride in a fluidized bed.